Add typed User-Agent client request support

[fewensa]

Summary

• Add HttpClient::user_agent that validates via rttp_protocol::user_agent::UserAgent and emits one canonical User-Agent field through header replace.
• Preserve automatic Mozilla/5.0 rttp/{version} when unset; typed values win over raw headers and the default without duplication.
• Re-export UserAgent, UserAgentMember, and UserAgentParseError; document behavior in the client README; cover HTTP/1.1, async, redirect, and HTTP/2 emission plus invalid-before-connect rejection.

Test plan

• cargo test -p rttp_client --all-features
• cargo test --all-features
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings

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